The present invention relates to heating members, heating elements, and to apparatuses useful for heating and cooling a workpiece that incorporate the heating members and heating elements, including bake/chill apparatuses and prime/chill apparatuses used in the manufacture of microelectronic devices. The heating members can be made to have a relatively high flatness, allowing improved efficiency and uniformity of heat transfer during processing.
The manufacture of many products requires precise control of temperature and temperature changes. For example, the manufacture of microelectronic devices such as integrated circuits, flat panel displays, thin film heads, and the like, involves applying a layer of some material, such as a photoresist, to the surface of a substrate such as a semiconductor wafer (in the case of integrated circuits). Photoresists, in particular, must be baked and then chilled to set or harden selected portions of the photoresist during processing. The baking and chilling steps must be precisely controlled within exacting temperature constraints to ensure that the selected portions of the photoresist properly set with good resolution. Nowadays, with the size of features becoming ever smaller and approaching sub-micron magnitudes, precise temperature and uniform heating of a workpiece become even more important.
Other products and processes involving precise temperature constraints include medical products and processes including drug preparation, instrument sterilization, and bioengineering; accelerated life testing methodologies; injection molding operations; piezoelectric devices; photographic film processing; material deposition processes such as sputtering and plating processes; micromachine manufacture; ink jet printing; fuel injection; and the like.
Baking and chilling operations for microelectronic devices typically involve cycling a workpiece through a desired temperature profile in which the workpiece is maintained at an elevated equilibrium temperature, chilled to a relatively cool equilibrium temperature, and/or subjected to temperature ramps of varying rates (in terms of xc2x0C./s) between equilibrium temperatures. To accomplish baking and chilling, some previous bake/chill operations have included separate bake and chill plates that require the use of a workpiece transport mechanism to physically lift and transfer the workpiece itself from one plate to another. This approach presents a number of drawbacks. First, workpiece temperature is not controlled during transfer between bake and chill plates. Second, the overall time required to complete the bake/chill process cannot be precisely controlled, because of the variable time required to move the workpiece to and from the respective plates. Third, the required movement takes time and thus reduces the throughput of the manufacturing process. Fourth, the cost of equipment includes the cost of components for handling the workpiece during transport from plate to plate. Fifth, the mechanical move from plate to plate introduces the possibility of contaminating of the workpiece. Thus, it is desirable to be able to accomplish both baking and chilling without having to physically lift and transport a workpiece from a heating member to a separate chill plate and vice versa.
A more recent approach of temperature control is described in U.S. Pat. No. 6,072,163, entitled xe2x80x9cCombination Bake/Chill Apparatus Incorporating Low Thermal Mass, Thermally Conductive Bakeplate.xe2x80x9d The patent describes methods that use a single apparatus having a low thermal mass heating member that supports a workpiece during both baking and chilling operations. While supporting the workpiece on one surface, the other surface of the heating member can be brought into and out of thermal contact with a relatively massive chill plate to easily switch between baking and chilling. A simple mechanism is used to physically separate the heating member and chill plate to effect rapid heating, or to join the heating member and the chill plate to effect rapid cooling. This approach eliminates the need to rely on workpiece handling to lift and transfer a workpiece from the heating member to a separate chill plate, and advantageously allows both chilling and baking to occur from a direction below the workpiece.
In bake and chill operations involving an apparatus with the combined ability to heat and cool, e.g., a bake/chill apparatus or a prime/chill apparatus, precise flatness has been found to be an important feature of the heating member. A typical gap between a supporting surface of a heating member and a workpiece supported by the heating member can be on the scale of several thousandths of an inch, e.g., less than six thousandths of an inch. It is important that the span of that gap be uniform over the entire area between the heating member and the workpiece so that heat is uniformly transferred between the two.
As an example of the effect of non-uniform heat transfer, consider the deposition of a reactive chemical layer such as a photoresist onto a microelectronic device. As noted above, finer and finer features are being placed on microelectronic devices, down to 0.13 microns and smaller. With continued reduction in feature size comes an attendant reduced tolerance for process non-uniformities. With smaller features, influences that in the past have had negligible effects on final quality of a processed workpiece become important. In the case of a photoresist used to produce such extremely small features, the temperature sensitivities of the photoresist may influence final product quality. Specifically, non-uniform temperatures across a layer of photoresist, even to a minute degree, can result in non-uniform thickness of a deposited photoresist layer or non-uniformity in the size of developed features, due to non-uniform solvent evaporation, or non-uniform reaction kinetics, e.g., development, chemical amplification, or photochemical reactions of a photoresist. These non-uniform processes, even if minutely small, can cause non-uniformities and imperfections in the details, e.g., feature sizes, of articles produced using the chemistries. Any methods of improving uniformity of heating a workpiece can improve product quality and reduce rejected products.
Another variable that can affect reaction kinetics, feature size, and uniformity, and ultimately the quality of manufactured products is the timing of heating and chilling processes. Many chemical reactions are temperature sensitive, meaning that they are designed to occur at a specific temperature. Optimum temperature control will involve a very rapid heating of a workpiece and its chemistry (e.g., photoresist) to the desired temperature, which will minimize the amount of time spent at a less-optimum temperature, and maximize the time spent reacting at the desired temperature. Overall, this increases the precision of the reaction and the uniformity of the reacted chemistry. Properties of a heating member that allow rapid, precise heating and cooling are particularly desirable. Agility in heating and cooling performance is desirable and very useful to provide high throughput and quality of workpieces.
It has been found that a high degree of uniformity in heating a workpiece can be achieved by selecting a low thermal mass heating member to have one or more of: an extreme degree of flatness of the supporting surface; high thermal conductivity of the thermally conductive layer; independent zones of temperature control; and a thermally conductive layer that has rigidity, stiffness, and thermal properties to achieve the desired flatness and thermal conductivity. Improved uniformity in heating a workpiece can improve the uniformity of chemical processing (e.g., solvent evaporation or chemical reactions) over the surface area of the workpiece, which improves the uniformity of feature sizes and ultimately increases product quality and yield.
Typical processes that have been used in constructing heating members have involved subjecting heating member materials to high pressures. In the past, heating members have been prepared from a number of layers such as metal layers and a heating element bonded together using high pressure and temperature. Many materials that have been used for layers of the heating member, such as metals like aluminum, have a relatively high flexibility. High pressures used in constructing the heating member have tended to adversely affect the heating member""s final flatness. As an example, bonding a heating element to a thin, flat aluminum sheet has been found to cause deformation of up to one one-hundredth of an inch in the aluminum sheet. The result is a reduced flatness or a warping of the heating member as a whole, which during use causes a non-uniform gap between the heating member and a workpiece. This in turn causes non-uniform heating of the workpiece by the heating member. As an additional challenge, the properties of metals include relatively low rigidity and relatively high flexibility, generally making it difficult to produce metal sheets that are both thin and flat to begin with. It has now been discovered that low thermal mass heating members can be constructed of relatively more rigid ceramic materials that do not suffer from the same dimensional instability and can be constructed to a relatively greater degree of flatness, and can at the same time be selected to have desired thermal properties such as low thermal mass and high, uniform thermal conductivity. Useful ceramic materials typically have a rigidity as measured by Young""s Modulus of at least 200 gigapascals, more preferably greater than about 400 gigapascals, which allows processing to preferred degrees of flatness. Preferred degrees of flatness may be less than about 0.01 inch, more preferably less than about 0.005 inch or even about 0.002 inch or less.
Preferred heating members can include a ceramic thermally conductive layer having a precise flatness, bonded to a heating element. A heating member can, for example, be constructed of a single rigid ceramic layer finished on one surface to a very high degree of flatness, and bonded on the other surface to a heating element. In another embodiment, the heating member can include two ceramic layers that sandwich a heating element between them. These heating members can be manufactured and assembled using techniques that do not significantly adversely reduce flatness of the ceramic thermally conducive layer, to produce a heating member of superior flatness which in use advantageously provides more uniform heat transfer than heating members with inferior flatness. The ceramic may also have additional advantageous properties for use in a combination bake/chill apparatus, such as one or more of a relatively low heat capacity (and therefore a low thermal mass) and high thermal conductivity.
One type of preferred ceramic material that can provide one or more of these advantages includes materials known as silicon carbides, especially silicon carbide having a purity in excess of 95 percent by weight, such as silicon carbide of greater than 97 weight percent purity, e.g., at least 98 or 99 weight percent purity, and even up to 99.999 weight percent purity or above. Such high purities offer improved uniformity and more uniform heat transfer properties. The thermally conductive layer may be made of other materials as well, especially other ceramics such as aluminum nitride (AlN), beryllium copper, beryllium, graphite foam, and like materials having preferred rigidity and thermal properties.
Other advantages of the inventive heating member may include one or more of the following. Improved flatness may eliminate the need to pull vacuum to pressure the workpiece against the heating member, which is sometimes used with other heating members (the use of vacuum may still be useful or desirable). This can reduce the complexity of the heating member itself and of an apparatus that uses the heating member. Also, the number of protuberances placed on a surface of the heating member to support a workpiece above the surface of the heating member may be reduced, or the need for protuberances may even be eliminated, allowing for more efficient heat transfer.
Preferred heating members of the invention can include a multi-layer heating element, meaning a heating element that includes heating element segments separated into two or more layers of the heating element. (Of course, single-layer heating elements may be useful.)
A preferred heating member may include the following: one or more thermally conductive layers (one on top including a workpiece supporting surface, and optionally a second thermally conductive layer opposite the heating element, which may be placed in thermal contact with a chill plate); a heating element optionally and preferably including multiple layers, each containing one or more heating element segment; adhesives; and may further additionally include optional components such as temperature sensors (optionally and preferably as a layer of the heating element), or others, as will be appreciated by the skilled artisan. A ground layer may be preferred to separate layers of an electrically resistive heating element segment from layers of temperature sensors.
Preferred embodiments of the invention can incorporate control equipment and methods that are able to precisely and accurately control temperatures of the heating member and the workpiece throughout heating and chilling steps, to make sure that the exacting temperature specifications for workpiece production are satisfied. For example, if heating or chilling rates as fast as 1xc2x0 C./s to 50xc2x0 C./s, preferably 5xc2x0 C./s to 15xc2x0 C./s, are used, the control system can be agile enough to control the workpiece temperature commensurately with such rapid temperature changes.
An aspect of the invention relates to a low thermal mass heating member that includes a thermally conductive layer having a supporting surface with a flatness of less than 0.01 inch. The thermally conductive layer also has an opposing surface in thermal contact with a heating element.
Another aspect of the invention relates to a low thermal mass heating member that includes a heating element in thermal contact with a thermally conductive layer, wherein the thermally conductive layer includes silicon carbide having a purity of at least about 98 percent by weight.
Another aspect of the invention relates to a low thermal mass heating member that includes a heating element in thermal contact with a thermally conductive layer, and the thermally conductive layer is aluminum nitride. The thermally conductive layer can consist of aluminum nitride, or can consist essentially of aluminum nitride.
Another aspect of the invention relates to a low thermal mass heating member that includes a multi-layer heating element in thermal contact with a thermally conductive layer. The multi-layer heating element includes multiple electrically resistive heating element segments, and at least two different layers of the multi-layer heating element each contain a heating element segment.
Yet another aspect of the invention relates to a multi-layer heating element. The multi-layer heating element includes multiple electrically resistive heating element segments, and at least two different layers of the multi-layer heating element each contain a heating element segment. The heating element can preferably be substantially flat and can be used for heating flat substrates.
Yet another aspect of the invention relates to a heating member that includes: a thermally conductive silicon carbide layer having a supporting surface with a flatness of less than 0.01 inch, wherein the silicon carbide has a thermal conductivity of at least 100 watts/(meter degree Kelvin); and a multi-layer heating element in thermal contact with an opposing surface of the silicon carbide layer, wherein the multi-layer heating element includes multiple electrically resistive heating element segments, and wherein at least two different layers of the multi-layer heating element each contain one or more heating element segments.
Still another aspect of the invention relates to an apparatus suitable for controlling the temperature of a workpiece. The apparatus includes: (a) a low thermal mass, thermally conductive heating member having a workpiece supporting surface adapted for supporting the workpiece in thermal contact with the heating member such that heat energy from the heating member can be transferred to the workpiece, the supporting surface of the heating member having a flatness of less than 0.01 inch; and (b) a high thermal mass chilling member. The apparatus supports the heating member and the chilling member in at least a first configuration in which the chilling member is in thermal contact with the heating member.
Still another aspect of the invention relates to an apparatus suitable for controlling the temperature of a workpiece. The apparatus includes: (a) a low thermal mass, thermally conductive, heating member that includes a ceramic thermally conductive layer having a workpiece supporting surface adapted for supporting the workpiece in thermal contact with the heating member such that heat energy from the heating member can be transferred to the workpiece, the ceramic layer being silicon carbide of at least about 98 percent by weight purity; and (b) a high thermal mass chilling member. The apparatus supports the heating member and the chilling member in at least a first configuration in which the chilling member is in thermal contact with the heating member.
Still a further aspect of the invention relates to an apparatus suitable for controlling the temperature of a workpiece. The apparatus includes: (a) a low thermal mass, thermally conductive, heating member comprising a thermally conductive layer having a workpiece supporting surface and an opposing surface, the workpiece supporting surface being adapted for supporting the workpiece in thermal contact with the heating member such that heat energy from the heating member can be transferred to the workpiece, the opposing surface being in thermal contact with a multi-layer heating element that includes multiple electrically resistive heating element segments, wherein at least two different layers of the multi-layer heating element each contain a heating element segment; and (b) a high thermal mass chilling member. The apparatus supports the heating member and the chilling member in at least a first configuration in which the chilling member is in thermal contact with the heating member.
Still another aspect of the invention relates to a combination bake/chill apparatus that includes a low thermal mass heating member having a silicon carbide thermally conductive layer of at least about 98 weight percent silicon carbide and having a surface having a flatness of less than 0.01 inch.
Still another aspect of the invention relates to a combination prime/chill apparatus that includes a low thermal mass heating member having an aluminum nitride thermally conductive layer comprising a surface having a flatness of less than 0.01 inch.